I. Field of the Invention
The invention concerns the prevention or treatment of inflammatory bowel disease by administering an interferon-gamma (IFN-γ) inhibitor.
II. Description of Background and Related Art
Inflammatory bowel disease (IBD) is a collective term for ulcerative colitis (UC) and Crohn's disease, which are considered as two different entities, but have many common features and probably share at least some pathologic mechanisms. There is sufficient overlap in the diagnostic criteria for UC and CD that it is sometimes impossible to say which a given patient has; however, the type of lesion typically seen is different, as is the localization. UC mostly appears in the colon, proximal to the rectum, and the characteristic lesion is a superficial ulcer of the mucosa; CD can appear anywhere in the bowel, with occasional involvement of stomach, esophagus and duodenum, and the lesions are usually described as extensive linear fissures.
The aetiology of these diseases is unknown and the initial lesion has not been clearly defined; however, patchy necrosis of the surface epithelium, focal accumulations of leukocytes adjacent to glandular crypts, and an increased number of intraepithelial lymphocytes and certain macrophage subsets have been described as putative early changes, especially in Crohn's disease.
The current therapy of IBD usually involves the administration of antiinflammatory or immunosuppressive agents, such as sulfasalazine, corticosteroids, 6-mercaptopurine/azathioprine, or cyclosporine, which usually bring only partial results. If anti-inflammatory/immunosuppressive therapies fail, colectomies are the last line of defense. About 30% of CD patients will need surgery within the first year after diagnosis. In the subsequent years, the rate is about 5% per year. Unfortunately, CD is characterized by a high rate of recurrence; about 5% of patients need a second surgery each year after initial surgery. In UC, a further reason for resorting to surgery is that the patients are known to be at much increased risk for developing colorectal cancer, starting 10-15 years after the diagnosis of ulcerative colitis. Presumably this is due to the recurrent cycles of injury to the epithelium, followed by regrowth, increasing the risk of transformation. Accordingly, colostomy is used as prophylaxis against the development of cancer in UC patients.
IBD is rather common, with a prevalence that is claimed to be in the range of 70-170 in a population of 100,000. In view of the apparent shortcomings of the present treatments, there is a great medical need for a nonsurgical approach, based upon a better understanding of the immunological reasons underlying this disorder.
A recent review of the characteristics of the mucosal immune system in IBD is by Brandtzaeg, P. et al. on pages 19-40 in Immunology of Gastrointestinal Disease, MacDonald, T. T. ed., Immunology and Medicine Series, Volume 19, Kluwer Academic Publishers, 1992.
Several attempts have been made to identify factors instrumental in the initiation of IBD; there have been reports on a genetically determined mucin defect in UC, and increased intestinal permeability and/or defective mucosal IgA system in Crohn's disease. There have further been persistent attempts to identify infectious agents associated with either UC or CD, with not much success. According to a recent, rather controversial theory, CD may be a vascular disease, characterized by localized tendency to thrombus formation leading to multifocal intestinal infarction and thereby causing the occurrence of early lesions. Altogether, the nature of the initial insult(s) resulting in IBD remains to be identified. However, there is considerable evidence that hyperactivation of the mucosal immune system in the gut through various immunopathological mechanisms may cause established IBD lesions.
The gut immune system is special in that the gut must absorb a vast amount of is potentially antigenic material (food proteins) without reacting to any of it, and must control reactions to non-pathogenic organisms such as normal gut flora without losing the ability to react to abnormal, replicating organisms. The regulatory mechanisms that allow this kind of selective response are almost completely unknown. It is also unclear whether IBD results from an appropriate immune response to an abnormally persistent antigen, or an inappropriate response to a normal antigen.
The major lymphocytic tissues in the small intestine are the so called Peyer's patches (PP). Unlike the lymph node, PP do not have a capsule of afferent lymphatics. The epithelium over the PP lacks the crypts and villi of normal gut epithelium and is referred to as follicle associated epithelium (FAE) containing cells called M cells. These are the major route of antigen transfer into the PP, and allow for direct sampling of antigen from the gut lumen by pinocytosis. Antigen is transported from the epithelium and presented to immunocompetent B cells, macrophages and dendritic cells in the underlying area. The colon has similar lymphoid arrangements called the lymphoid follicles. Lymphoid follicles are not identical to PP, but also have specialized epithelium containing M cells, and probably function as antigen presenting sites.
Underneath the epithelium there is a tissue called the lamina propria which forms the core of the villus and is densely infiltrated with lymphocytes bearing homing receptors which selectively bind to the mucosal lymphoid high endothelium. B cells comprise about 50% of the lymphocytes in the lamina propria of the gut, whereas the other half of lymphocytes are CD3+ T cells most of which are also CD4+. In the normal intestine, most of the B cells in the lamina propria are IgA+, although IgM-, IgG- and IgD-expressing cells are also found. Most of the immunoglobulin secreted into the intestine is IgA, and half of that is IgA-2, in contrast to the lymph nodes where most of the secreted IgA is of the IgA-1 isotype. The abundance of IgA antibodies is probably crucial for immunological homeostasis within the lamina propria. IgA antibodies lack potent effector functions such as complement activation, and may therefore block non-specific biological amplification mechanisms triggered by locally produced or serum-derived IgG antibodies.
As already mentioned, CD3+ T cells comprise approximately half of the lymphocytes in the lamina propria. This phenotype is also prevalent in human PP, and specifically in the interfollicular zones surrounding the high endothelial venules (HEV). In contrast, CD8+ T lymphocytes are predominant in the epithelium of humans.
Although it is not clear how inductive and suppressive immunoregulatory mechanisms are achieved in the gut, the lamina propria and epithelium, along with the organized lymphoepithelial nodules and the larger lymphoid aggregates, e.g. PP, are probably all involved in a complex manner.
The established mucosal IBD lesions are dominated by immunoglobulin-producing cells, both in UC and in CD. However, while the IgA- and IgM-expressing cell populations only increase several times as compared to normal mucosa, there is a disproportionate rise in the number of IgG-producing immunocytes. The actual number depends on the severity of the disease, but both UC and CD are characterized by a dramatically increased IgG production, including selective increases in the levels of specific IgG isotypes in both the intestinal mucosa and peripheral blood, and consequently by a remarkable decrease in the IgA/IgG ratio [MacDermott, R. P. et al., Gastroenterology 96, 764-768 (1989)].
It has been tentatively suggested that the selective increase in the production of IgG might reflect an immune response to one or several antigens, as well as the balance of cytokines and other regulatory factors, such as transforming growth factor β (TGF-β), that modulate immunoglobulin production in different populations of B cells [Podolsky, D. K., New England J. Med. 325, 928-937 (1992)]; however, there is no satisfactory explanation for this phenomenon as of yet.
Changes in the major subsets of the T cells population have also been observed in IBD. In CD, there is evidence that there are more memory cells than normal (lacking the CD45RA marker) and increased EL-2R expressing (activated) cells. CD4+ T-cells in the lamina propria seem to be increased relative to CD8+ T-cells in UC. It has been observed that the expression of certain cytokines is increased in IBD. This includes increased expression of interleukin-1 (IL-1), interleukin-6 (IL-6), altered expression of interleukin-2 (IL-2) and its receptor in both tissue and the circulation [Mahida, Y. R. et al., Gut 30, 838-838 (1989); Kusugami, K. et al., Gastroenterology 97, 1-9 (1989); Ligumsky, M. et al., Gut 31, 686-689 (1990)]. Curiously, lower levels of IL-2 an,d IL-2 receptors have been reported in tissue from certain patients diagnosed with CD (Kusugami et. al., supra). An IL-1 receptor antagonist has been described to reduce the severity of inflammation in a rabbit model of colitis [Cominelli, F. et al., J. Clin. Invest. 86 972-980 (1990)]. A remarkably intensified epithelial expression of human leukocyte antigen complex DR(HLA-DR)in both UC and CD has led to the proposal that various cytokines are released locally from activated T cells [Selby, W. S. et al., Clin. Exp. Immunol. 53, 614-618 (1983); Brandtzaeg, P. et al. Ann. Gastroenterol. Hepatol. 21, 201-220 (1985); Rognum, T. O. et al., Gut 23, 123-133 (1982); Fais, S. et al., Clin. Exp. Immunol. 605-612 (1987)]. Although both IFN-γ and tumor necrosis factor α (TNF-α) are capable of enhancing epithelial HLA-DR on intestinal epithelium, difficulties have been encountered in demonstrating the production of IFN-γ in the IBD lesion [MacDonald, T. T. et al., Clin. Exp. Immunol. 81, 301-305 (1990)]. On the other hand, a raised number of cells producing TNF-α has been observed for both UC and CD lesions (MacDonald et al, supra).
In conclusion, whereas the relatively reduced IgA production and the striking increase of IgG-producing cells in IBD may reflect the establishment of a local immune defense mechanism, the causative factors for these changes have not yet been determined. Similarly, despite suggestions that certain cytokines and cytokine receptors may play a role in the development of IBD, their individual roles and complex interactions are not well understood.
It is an object of the invention to provide a method for the prophylaxis or treatment of inflammatory bowel disease (IBD), including ulcerative colitis (UC) and Crohn's disease (CD).
It is a further object to provide bispecific molecules comprising an IFN-γ inhibitor and a further specificity to a target involved in the initiation or development of IBD.
It is yet another object to provide a method for the use of IFN-γ inhibitors in the preparation of pharmaceutical compositions suitable for the prophylaxis or treatment of disorders that involve a reduction in the percentage of IgA-producing immunocytes, such as IBD.
These and further objects of the present invention will be apparent for one skilled in the art.